Polyester film for capacitors

ABSTRACT

A film which is a biaxially oriented film comprising polyethylene-2,6-naphthalenedicaboxylate as a principal component, having a thermal strain ratio RMD (150) in the machine direction of the film at a temperature of 150° C. is −1.5%≦RMD (150)≦0.0% when the thermal strain ratio RMD (T) in the machine direction of the film at a temperature T° C. based on the length LMD (t) in the machine direction (MD) of the film at a temperature t° C. is defined by
 
 RMD ( T )={[ LMD ( T )− LMD (35)]/ LMD (35)}×100(%),
 
excellent in insulation characteristics without lowering processing characteristics, especially the characteristics during hot-pressing and suitable as a dielectric for capacitors.

TECHNICAL FIELD

The present invention relates to a polyester film for capacitors.

BACKGROUND ART

Miniaturization and larger capacity are demanded according to theminiaturization of electronic and electrical equipment in capacitors andan improvement in insulating characteristics is simultaneously demandedaccording to the higher voltage the capacitors are used. In response tothe demands, in film capacitors, studies have been made on thinner filmsas a dielectric and on a reduction in pinholes present in films. Theelectrostatic capacity based on unit volume of the film as thedielectric is in inverse proportion to the square of the film thicknessand in proportion to the permittivity of the dielectric. Although theminiaturization and larger capacity of capacitors can be achieved infilm capacitors by forming thinner films which are dielectrics,operating efficiency is lowered in processing steps of capacitors, andespecially the operating efficiency is lowered in steps of metal vapordeposition on films and slit processing and winding for stacking a filmof a piece of capacitor. A proposal was made for a method forsuppressing the deterioration in operating efficiency by adding specificinert fine particles as a lubricant into apolyethylene-2,6-naphthalenedicarboxylate film [JP-A (JP-A refers toJapanese Unexamined Patent Publication) No. 10-294237].

DISCLOSURE OF THE INVENTION

The methods mentioned above are excellent methods in that excellentoperating efficiency can be obtained in processing steps of capacitorswhile forming thinner films; however, defective insulatingcharacteristics due to additives, especially lubricants contained in thefilms may be caused, and the resulting films are still insufficient asfilms for capacitors. There is a tendency to increase the temperatureand pressure of hot-pressing after winding for stacking a film of apiece of capacitor, and insulation resistance or withstand voltage asthe piece of capacitor may be lowered from that of a film simplematerial.

An object of the present invention is to solve the problems of the priorart and to provide a polyester film for capacitors comprisingpolyethylene-2,6-naphthalenedicarboxylate as a principal component andexcellent in insulating characteristics and processability.

The present invention is a constitution of the following item 1 andincludes items 2 to 14 as preferred modes.

-   item 1. A polyester film for capacitors which is a biaxially    oriented film comprising polyethylene-2,6-naphthalenedicarboxylate    as a principal component and having a thermal strain ratio RMD (150)    of the film in the machine direction at a temperature of 150° C. is    −1.5%≦RMD(150)≦−0.0%    when the thermal strain ratio RMD (T) in the machine direction at a    temperature T° C. based on the length LMD (t) in the machine    direction (MD) of the film at a temperature t ° C. is defined by    RMD(T)={[LMD(T)−LMD(35)]/LMD(35)}×100(%).-   item 2. The polyester film for capacitors according to item 1,    characterized in that the thermal strain ratio RTD (150) in the    transverse direction of the film at a temperature of 150° C. is    −1.0%≦RTD(150)≦0.0%    when the thermal strain ratio RTD (T) in the machine direction of    the film at a temperature T ° C. based on the length LTD (t) in the    transverse direction (TD) of the film at a temperature t ° C. is    defined by    RTD(T)={[LTD(T)−LTD(35)]/LTD(35)}×100(%).-   item 3. The polyester film for capacitors according to item 2,    characterized in that the thermal strain ratios RMD (210) and    RTD (210) at a temperature of 210° C. are     −3.5%≦RMD(210)≦0.0% and    −3.5%≦RTD(210)≦0.0%.-   item 4. The polyester film for capacitors according to item 3,    characterized in that the ratio of the 5% strain strength in the    machine direction of the film to the 5% strain strength in the    transverse direction of the film is 0.90 or more and 1.40 or less.-   item 5. The polyester film for capacitors according to any of items    1 to 4, characterized in that the number of fry specks having an    average diameter exceeding 60 μm is 20 fly specks/m² or less.-   item 6. The polyester film for capacitors according to any of items    1 to 5 characterized in that the number of fry specks having an    average diameter exceeding 30 μm is 10 fly specks/m² or less.-   item 7. The polyester film for capacitors according to any of items    1 to 6, characterized in that the number of coarse particles present    in the film and having the maximum diameter exceeding 40 μm is 10    coarse particles/m² or less.-   item 8. The polyester film for capacitors according to item 5,    characterized in that 0.03 to 2% by weight of calcium carbonate    particles having an average particle diameter of 0.2 to 5 μm is    contained and 0.03 to 1% by weight of platy aluminum silicate    particles having an average particle diameter of 0.1 to 2 μm is    contained.-   item 9. The polyester film for capacitors according to item 8,    characterized in that the number of coarse particles having the    maximum diameter exceeding 35 μm is 10 coarse particles/m² or less.-   item 10. The polyester film for capacitors according to item 7,    characterized in that a porous silica and a spherical silica are    contained, the porous silica particles have an average particle    diameter of 0.5 to 5 μm, the spherical silica particles have an    average particle diameter of 0.05 to 1.5 μm and less than the film    thickness, the spherical silica particles further have a particle    diameter ratio of 1.0 to 1.2, the content of the porous silica    particles is 0.05 to 2% by weight and the content of the spherical    silica particles is 0.01 to 1% by weight.-   item 11. The polyester film for capacitors according to item 10,    characterized in that the number of fly specks having an average    diameter exceeding 55 μm is 15 fly specks/m² or less.-   item 12. The polyester film for capacitors according to item 6,    characterized in, that two kinds of spherical silica particles    different in average diameter are contained.-   item 13. The polyester film for capacitors according to item 12,    characterized in that the two kinds of spherical silica different in    average particle diameter are spherical silica particles (A) having    an average particle diameter of 0.5 to 3.0 μm and spherical silica    particles (B) having an average particle diameter of 0.01 to 1.5 μm,    the spherical silica particles (A) and the spherical silica    particles (B) each have a particle diameter ratio of 1.0 to 1.2, the    content of the spherical silica particles (A) is 0.03 to 1.5% by    weight and the content of the spherical silica particles (B) is 0.05    to 2% by weight.-   item 14. The polyester film for capacitors according to any of items    9, 11 and 13, characterized in that the film is produced according    to a simultaneous biaxial stretching method.

The present invention will be explained in more detail.

<<Polyethylene-2,6-naphthalenedicarboxylate>>

The polyester film for capacitors of the present invention comprisespolyethylene-2,6-naphthalenedicarboxylate as a principal component. Onthe other hand, the conventional polyester film for capacitors comprisespolyethylene terephthalate as a principal component and has apossibility of increasing the dielectric dissipation factor in atemperature region of 80° C. or above, self-heating by the dielectricloss and causing thermal runaway. Thereby, the upper limit of theservice temperature of the conventional polyester film for capacitors asa capacitor is limited to about 80° C. In contrast to this, thepolyethylene-2,6-naphthalenedicarboxylate in the present inventionincreases the dielectric dissipation factor from the vicinity of 120° C.and the upper limit of the service temperature, therefore, is about 120°C. Accordingly, the polyester film for capacitors of the presentinvention can be used in environment at a higher temperature than thatof the conventional polyester film for capacitors.

The polyethylene-2,6-naphthalenedicarboxylate used in the presentinvention is a polyester in which the main dicarboxylic acid componentis naphthalenedicarboxylic acid and the main glycol component isethylene glycol. Examples of the naphthalenedicarboxylic acid include2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid and1,5-naphthalenedicarboxylic acid. Above all, 2,6-naphthalenedicarboxylicacid is preferable.

In the present invention, “comprisespolyethylene-2,6-naphthalenedicarboxylate as a principal component”means that at least 90 mole %, preferably at least 95 mole % of thewhole recurring units in the constituent component of the polymerconstituting the film is ethylene-2,6-naphthalenedicarboxylate.

A polymer which is capable of ensuring insulating characteristics,mechanical characteristics and thermal dimensional stability may be usedas the polymer constituting the polyester film for capacitors of thepresent invention and the polymer may be a copolymer or a blendcomprising the polyethylene-2,6-naphthalenedicarboxylate as theprincipal component.

When the copolymer is used, a compound having two ester-formingfunctional groups in the molecule can be used as a copolymerizationcomponent constituting the copolymer other than theethylene-2,6-naphthalenedicarboxylate which is the principal component.Examples of the compound include dicarboxylic acids such as oxalic acid,adipic acid, phthalic acid, sebacic acid, dodecanedicarboxylic acid,isophthalic acid, terephthalic acid, 1,4-cyclohexanedicarboxylic acid,4,4′-diphenyldicarboxylic acid, phenylindanedicarboxylic acid,2,7-naphthalenedicarboxylic acid, tetralindicarboxylic acid,decalinedicarboxylic acid and diphenyl etherdicarboxylic acid;hydroxycarboxylic acids such as p-hydroxybenzoic acid andp-hydroxyethoxybenzoic acid; dihydric alcohols such as propylene glycol,trimethylene glycol, tetramethylene glycol, hexamethylene glycol,cyclohexanemethylene glycol, neopentyl glycol, ethylene oxide adducts ofbisphenol sulfone, ethylene oxide adducts of bisphenol A, diethyleneglycol, polyethylene oxide glycol and the like. One kind of thecompounds may be used and two or more kinds may be used in combination.Preferable dicarboxylic acids in the components are isophthalic acid,terephthahic acid, 4,4′-diphenyldicarboxylic acid,2,7-naphthalenedicarboxylic acid and p-hydroxybenzoic acid andpreferable glycol components are trimethylene glycol, hexamethyleneglycol, neopentyl glycol and ethylene oxide adducts of bisphenolsulfone.

A part or all of terminal hydroxy groups and/or carboxy groups of thepolyethylene-2,6-naphthalenedicarboxylate may be terminated with amonofunctional compound, for example benzoic acid or amethoxypolyalkylene glycol or may be a copolymer with a trace amount ofan ester-forming compound having three or more functional groups such asglycerol or pentaerythritol within the range so as to provide asubstantially linear polymer.

The polymer of the polyester film for capacitors of the presentinvention may be a blend; however, thepolyethylene-2,6-naphthalenedicarboxylate may contain an organic polymerwhen the polymer is the blend. In this case, 90 mole % or more of thepolyethylene-2,6-naphthalenedicarboxylate is preferably contained in thepolymer.

Examples of the organic polymer include polyethylene terephthalate,polyethylene isophthalate, polytrimethylene terephthalate, polyethylene4,4′-tetramethylenediphenyldicarboxylate,polyethylene-2,7-naphthalenedicarboxylate,polytrimethylene-2,6-naphthalenedicarboxylate,polyneopentylene-2,6-naphthalenedicarboxylate andpoly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-naphthalenedicarboxylate.Above all, polyethylene isophthalate, polyrtimethylene terephthalate,polytrimethylene-2,6-naphthalenediacrboxylate andpoly(bis(4-ethyleneoxyphenyl)sulfone)-2,6-napthalenedicarboxylate arepreferable.

One kind of the organic polymer may be used and two or more kinds may beused in combination.

Polyethylene-2,6-naphthalenedicarboxylate can be produced by aconventionally known method. For example, thepolyethylene-2,6-naphthalenedicarboxylate can be produced according to amethod for directly providing a polyester having a low degree ofpolymerization by a reaction of a dicarboxylic acid with a glycol or canbe produced by carrying out transesterification of a lower alkyl esterof a dicarboxylic acid with a glycol using a transesterificationcatalyst and then conducting polymerization using a polymerizationcatalyst. For example, one or two or more kinds of compounds containingsodium, potassium, magnesium, calcium, zinc, strontium, titanium,zirconium, manganese or cobalt can be used as the transesterificationcatalyst. Examples of the polymerization catalyst include antimonycompounds such as antimony trioxide or antimony pentoxide, germaniumcompounds such as germanium dioxide or titanium compounds such astetraethyl titanate, tetrapropyl titanate, tetraphenyl titanate orpartial hydrolyzates thereof, ammonium titanyl oxalate, potassiumtitanyl oxalate or titanium trisacetylacetonate.

When polymerization is carried out through transesterification, aphosphorus compound such as trimethyl phosphate, triethyl phosphate,tri-n-butyl phosphate or orthophosphoric acid is usually added for thepurpose of deactivating the transesterification catalyst beforepolymerizing reaction. In the present invention, it is preferable thatthe content of phosphorus element in thepolyethylene-2,6-naphthalenedicarboxylate is 20 to 100 ppm from theviewpoint of thermal stability of the polyester.

Furthermore, the polyester may be formed into chips after meltpolymerization and subjected to solid-phase polymerization under heatingdecompression or in an inert gas stream such as nitrogen.

The intrinsic viscosity of the polyester used as the polyester film forcapacitors of the present invention is preferably 0.40 to 0.90 dl/g,more preferably 0.43 to 0.85 dl/g, especially preferably 0.45 to 0.80dl/g when the film of the polyester is used as a sample. When theintrinsic viscosity is less than 0.40 dl/g, it is undesirable that thefilm becomes brittle and breakage during the formation of the film tendsto occur to readily cause the breakage of the film in transportation inprocessing steps of the capacitors. On the other hand, when theintrinsic viscosity exceeds 0.90 dl/g, it is undesirable that theintrinsic viscosity of the polymer must be considerably increased, along time is required for polymerization according to a usual synthetictechnique and the productivity is deteriorated.

<<Thermal Strain Ratio RMD (150) and RTD (150)>>

The thermal strain ratio RMD (T) in the machine direction of a film at atemperature T ° C. is defined byRMD(T)={[LMD(T)−LMD(35)]/LMD(35)}×100(%)based on the length LMD (t) in the machine direction (MD) of the film ata temperature t ° C., wherein the LMD (35) is the length in the machinedirection (MD) of the film at a temperature of 35° C. Namely, in thepresent invention, the thermal strain ratio of the film is defined by adegree of change in the film length changing with temperature based onthe film length at a temperature of 35° C. The film of the presentinvention is characterized in that the thermal strain ratio RMD (150) inthe machine direction of the film at a temperature of 150° C. is−1.5%≦RMD(150)≦0.0

When the thermal strain ratio RMD (150) at 150° C. is less than −1.5%,the film shrinks in a process for providing the surface of the film witha metal layer and the planarity is lowered to thereby deteriorate thedielectric loss of capacitors. On the other hand, when the RMD (150)exceeds 0.0%, a defective shape is sometimes caused during hot-pressingand insulation resistance is lowered.

In the present invention, the thermal strain ratio RTD (150) in thetransverse direction of the film at a temperature of 150° C. ispreferably −1.0%≦RTD (150)≦0.0% when the thermal strain ratio RTD (T) inthe machine direction of the film at a temperature T ° C. based on thelength LTD (t) in the transverse direction (TD) of the film at atemperature t ° C. is defined byRTD(T)={[LTD(T)−LTD(35)]/LTD(35)}×100(%).

When the RTD (150) is less than −1.0%, the film shrinks in a process forproviding the surface of the film with a metal layer and theelectrostatic capacity is sometimes insufficient. On the other hand,when the RTD (150) exceeds 0.0%, it is undesirable that the metallizedfilm causes curling after providing the surface with the metal layer andthe yield is sometimes lowered.

<<Thermal Strain Ratios RMD (210) and RTD (210)>>

In the present invention, the thermal strain ratios RMD (210) and RTD(210) at a temperature of 210° C. are preferably−3.5%≦RMD(210)≦0.0% and−3.5%≦RTD(210)≦0.0%.When at least one of the thermal strain ratio RMD (210) and the thermalstrain ratio RTD (210) at 210° C. is less than −3.5% or exceeds 0.0%, itis undesirable that defective contact is caused by deformation of thefilm and capacity becomes sometimes insufficient during metallicon(thermal spraying of an electrode metal on both end faces of a piece ofcapacitor).

<<Machine Direction/Transverse Direction Ratio of 5% Strain Strength>>

In the present invention, the ratio of 5% strain strength in the machinedirection to the 5% strain strength in the transverse direction of thefilm, i.e. a value obtained by dividing the 5% strain strength in themachine direction by the 5% strain strength in the transverse directionis preferably 0.90 to 1.40. When the value is less than 0.90, it isundesirable that the film readily elongates in the machine directionduring processing. When the value exceeds 1.40, it is undesirable thatthe thermal strain ratio RMD (150) in the machine direction (MD) at 150°C. is sometimes less than −1.5%.

<<Center Line Average Height (Ra)>>

The center line average height (Ra) of the film for capacitors of thepresent invention is preferably 35 to 90 nm. When the center lineaverage height (Ra) is less than 35 nm, it is undesirable thatslipperiness and air escaping property of the film are inferior, thewinding of the film into a roll becomes extremely difficult, collapsingproperty of the piece of capacitor during hot-pressing is inferior anddefective shape and lowering of insulation resistance are sometimescaused. The defective shape herein described is caused by defectiveslipperiness and is a phenomenon in which a collapse mark in the centerof the piece of capacitor does not become rectilinear and a strain iscaused or dumbbelled voids remain at both ends of the collapse mark. Onthe other hand, it is necessary to add a lubricant having a largeparticle diameter or increase the amount of the added lubricant in orderto obtain a film having a center line average height (Ra) exceeding 90nm; however, it is undesirable that the thickness of the polyester filmfor capacitors of the present invention is small and the breakagefrequency during the formation of the film is resultantly increased tomarkedly lower the productivity or it is undesirable that the breakdownvoltage of the capacitors is lowered.

In addition, when the polyester film for capacitors of the presentinvention contains calcium carbonate particles and platy ammoniumsilicate particles, the preferable range of the center line averageheight (Ra) is 40 to 80 nm and the far more preferable range is 45 to 80nm with the especially preferable range of 50 to 77 nm.

When the polyester film for capacitors of the present invention containsporous silica and spherical silica, the more preferable range of thecenter line average height (Ra) is 40 to 90 nm and the far morepreferable range is 45 to 87 nm with the especially preferable range of50 to 85 nm.

When the polyester film for capacitors of the present invention containstwo kinds of spherical silica different in average particle diameter,the more preferable range of the center line average height (Ra) is 35to 75 nm and the far more preferable range is 40 to 70 nm with theespecially preferable range of 45 to 67 nm.

<<Ten-Point Average Roughness (Rz)>>

The ten-point average roughness (Rz) of the polyester film forcapacitors of the present invention is preferably 900 to 1800 nm. Whenthe ten-point average roughness is less than 900 nm, it is undesirablethat the slipperiness and air escaping property of the film areinferior, the winding of the film into a roll becomes extremelydifficult, collapsing property of the piece of capacitor duringhot-pressing is inferior and the defective shape and lowering ofinsulation resistance are sometimes caused. It is necessary to add alubricant having a large particle diameter or increase the amount of theadded lubricant in order to obtain a film having a ten-point averageroughness exceeding 1800 nm; however, it is undesirable that thethickness of the polyester film for capacitors of the present inventionis small and the breakage frequency during the formation of the film isresultantly increased to markedly lower the productivity or it isundesirable that the breakdown voltage of the capacitors is lowered inthis case.

When the polyester film for capacitors of the present invention containsporous silica and spherical silica, the more preferable range of theten-point average roughness is 1000 to 1800 nm and the more preferablerange is 1100 to 1750 nm with the especially preferable range of 1200 to1700 nm.

When the polyester film for capacitors of the present invention containstwo kinds of spherical silica different in average particle diameter,the more preferable range of the ten-point average roughness is 900 to1500 nm and the far more preferable range is 1000 to 1450 nm with theespecially preferable range of 1100 to 1400 nm.

<<Thickness>>

The thickness of the polyester film for capacitors of the presentinvention is preferably 0.3 to 10 μm, more preferably 0.3 to 7.0 μm, farmore preferably 0.4 to 7.0 μm, still far more preferably 0.4 to 6.0 μm,especially preferably 0.5 to 5.0 μm. When the film thickness is lessthan 0.3 μm, it is undesirable that the breakage frequently occursduring the film formation and the film formation is difficult. When thefilm thickness exceeds 10 μm, a phenomenon of lowering of insulatingcharacteristics does not occur and there is no problem about lowering ofdielectric strength by pressing because the sufficient resin thicknesscan be maintained even if fly specks are present. Therefore, this casedoes not become an object of the present invention.

<<Dispersion of Thickness>>

The dispersion of thickness of the polyester film for capacitors of thepresent invention in an optional site on the film is preferably 25% orless, more preferably 20% or less, especially preferably 15% or lessbased on the film thickness. When the dispersion of thickness based onthe thickness of the film exceeds 25%, it is undesirable that thedispersion of performances as capacitors is caused by the dispersion ofthe thickness when the film as a thin film for a dielectric of thecapacitors is laminated several times for use.

<<Density>>

The density of the polyester film for capacitors of the presentinvention is preferably 1.340 to 1.361 g/cm³, more preferably 1.345 to1.357 g/cm³. When the density is less than 1.340 g/cm³, it isundesirable that a dispersion of capacitor performances is caused tobring about the lowering of processing yield. When the density exceeds1.361 g/cm³, it is undesirable that the crystallinity becomes too highto lose the toughness of the film and the breakage frequency duringtransportation of the film or slit processing is resultantly increased.

<<Calcium Carbonate Particles and Platy Aluminum Silicate Particles>>

When calcium carbonate particles are contained in the polyester film forcapacitors of the present invention, the average particle diameter ofthe calcium carbonate particles is preferably 0.2 to 5 μm, morepreferably 0.3 to 4 μm, especially preferably 0.5 to 3 μm from theviewpoint of the slipperiness and air escaping property of the film. Theamount of the added calcium carbonate particles is preferably 0.03 to 2%by weight, more preferably 0.05 to 1.5% by weight, especially preferably0.1 to 1% by weight.

Examples of the calcium carbonate in the present invention includecalcite crystals such as naturally occurring limestone and chalk(whiting) and precipitated calcium carbonate produced from limestone bya chemical method; argonite crystals obtained by reacting milk of limewith gaseous carbon dioxide at high temperatures, vaterite crystals anda combination thereof. Calcium carbonate heavy (calcite crystals)obtained by mechanically pulverizing limestone can also be used.

When platy aluminum silicate particles are contained in the polyesterfilm for capacitors of the present invention, the average particlediameter of the platy aluminum silicate particles is preferably 0.1 to 2μm, more preferably 0.3 to 1.7 μm, especially preferably 0.5 to 1.5 μm.The amount of the added platy aluminum silicate particles is preferably0.03 to 1% by weight, more preferably 0.06 to 0.8% by weight, especiallypreferably 0.1 to 0.7% by weight from the viewpoint of slipperiness ofthe film and handleability in the production process for capacitors.

In the present invention, the platy aluminum silicate refers to analuminosilicate and examples include kaolin clay comprising naturallyoccurring kaolin minerals. Furthermore, the kaolin clay may be subjectedto refining treatment such as washing with water.

When the polyester film for capacitors of the present invention containscalcium carbonate particles and platy aluminum silicate particles, thenumber of fly specks having an average diameter exceeding 60 μm ispreferably 20 fly specks/m² or less, more preferably 15 fly specks/m² orless, especially preferably 10 fly specks/m² or less. The fly specksmarkedly lowering insulating performances have an average diameterexceeding 60 μm and it is undesirable that insulating performances as adielectric of the capacitors are insufficient when the frequency ofpresence of the fly specks exceeds 20 fly specks/m².

Furthermore, the fly specks occur from additives (lubricants or thelike) or foreign materials contained in the resin as nuclei and arecomposed of the nuclei and parts (voids) in which the film thickness ofresins constituting the film formed in the periphery thereof is reduced.The average diameter of the fly specks is the average of the major axisand the minor axis of the fly specks.

<<Porous Silica Particles and Spherical Silica Particles>>

When porous silica particles are contained in the polyester film forcapacitors of the present invention, the porous silica particles arepreferably composed of aggregates of primary particles having an averageparticle diameter of 0.001 to 0.1 μm. The porous silica particlesexhibit a high affinity for polyethylene-2,6-naphthalenedicarboxylate;however, coarse particles are frequently present because the poroussilica particles are composed of the aggregates. The coarse particlescontained in the film cause lowering of performances of the polyesterfilm for capacitors. When the average particle diameter of the primaryparticles is less than 0.001 μm, it is undesirable that the particlesurface area is increased, particles resultantly tend to mutuallyaggregate and coarse aggregates are produced. When the average particlediameter of the primary particles exceeds 0.1 μm, it is undesirable thatthe porosity of the particles is lost, and the affinity forpolyethylene-2,6-naphthalenedicarboxylate is lost and voids readilyoccur around the lubricant to deteriorate insulating characteristics.

The average particle diameter of the primary particles can be measuredas follows. Individual particles of a powder to be a sample areinitially scattered. A metal vapor deposition film is then formed on thesurface at a thickness of 200 to 300 Å with a gold sputtering device.The resulting metal vapor deposition film is observed at 10000 to 30000times under a scanning type electron microscope and image processing iscarried out with Luzex 500 manufactured by Nippon Regulator Co., Ltd.Thereby, the average particle diameter can be measured.

The average particle diameter as the aggregates of the porous silicaparticles is preferably 0.5 to 5 μm, more preferably 0.7 to 4.0 μm,especially preferably 1.0 to 3.0 μm from the viewpoint of slipperinessand deairing properties of the film. The amount of the added poroussilica particles is preferably 0.05 to 2% by weight, more preferably0.07 to 1.8% by weight, especially preferably 0.1 to 1.5% by weight.

The pore volume of the porous silica particles is preferably 0.5 to 2.0ml/g, more preferably 0.6 to 1.8 ml/g. When the pore volume is less than0.5 ml/g, it is undesirable that the porosity is poor and the affinityfor polyethylene-2,6-naphthalenedicarboxylate is lost. When the porevolume exceeds 2.0 ml/g, it is undesirable that the aggregation tends tooccur and the adjustment of the particle diameter becomes difficult.

When the spherical silica particles are contained in the polyester filmfor capacitors of the present invention, the deterioration inslipperiness of the film due to lowering of particle diameter bydisintegration of the porous silica particles in an extruding step or arecovering step in production of the film can be suppressed thereby.

The average particle diameter of the spherical silica particles ispreferably 0.05 to 1.5 μm. The average particle diameter is preferablysmaller than the film thickness in order to stably produce the film.Furthermore, the average particle diameter is preferably 90% or lessbased on the film thickness and especially preferably 80% or less basedon the film thickness.

The amount of the added spherical silica particles is preferably 0.01 to1% by weight, more preferably 0.03 to 0.9% by weight from the viewpointof slipperiness and windability of the film and handleability in theproduction process for the capacitors. The particle diameter ratio(major axis/minor axis) of the spherical silica particles is preferably1.0 to 1.2.

The number of fly specks having an average diameter exceeding 55 μm ispreferably 15 fly specks/m² or less, more preferably 10 fly specks/m² orless, especially preferably 8 fly specks/m² or less. The averagediameter of fly specks markedly lowering insulating performances exceeds55 μm. When the frequency of presence of the fly specks exceeds 15 flyspecks/m², it is undesirable that the insulating performances as adielectric of capacitors are insufficient.

<<Spherical Silica Particles (A) and (B)>>

When spherical silica particles are contained in the polyester film forcapacitors of the present invention, 0.03 to 1.5% by weight of thespherical silica particles (A) having an average particle diameter of0.5 to 3.0 μm is preferably contained and 0.05 to 2% by weight of thespherical silica particles (B) having an average particle diameter of0.01 to 1.5 μm is preferably contained. The spherical silica particles(A) and the spherical silica particles (B) each have a particle diameterratio (aspect ratio) of preferably 1.0 to 1.2. The average particlediameter of the spherical silica particles (A) is preferably larger thanthat of the spherical silica particles (B). Furthermore, the averageparticle diameter of the spherical silica particles (A) is morepreferably 0.8 to 2.5 μm and the average particle diameter of thespherical silica particles (B) is more preferably 0.01 to 1.2 μm.

Since the two kinds of the spherical silica particles each have aparticle diameter ratio of 1.0 to 1.2, the shape of the individual fineparticles is extremely close to a sphere and is characterized in thatthe spherical silica particles are remarkably different from silica fineparticles which are heretofore known as a lubricant and are ultrafinemassive particles of about 10 nm or aggregates (agglomerate particles)of about 0.5 μm formed from the ultrafine massive particles byaggregation. When the particle diameter ratio exceeds 1.2, thecontribution to surface roughness is reduced and voids tend to form inthe interface between the particles and the polymer.

The particle diameter ratio (aspect ratio) herein is determined byParticle diameter ratio=average major axis of spherical silica fineparticles/average minor axis of spherical silica fine particles.Furthermore, the average major axis and the average minor axis aredetermined by the average of numerical values of the major axis andminor axis of optionally selected sample particles.

When the average particle diameter of the spherical silica particles (A)is less than 0.5 μm, improving effects on the slipperiness or operatingefficiency of the film are insufficient. On the other hand, when theaverage particle diameter exceeds 3.0 μm, it is undesirable that thebreaking strength of the film is lowered to easily break the film andinsulation defects are sometimes caused. When the average particlediameter of the spherical silica particles (B) is less than 0.01 μm,improving effects on slipperiness or operating efficiency of the filmare insufficient. On the other hand, when the average particle diameterexceeds 1.5 μm, it is undesirable that the surface of the film isexcessively roughened, and that the film surface between largeprotrusions cannot be moderately roughened and the collapse load duringpressing cannot be reduced.

Thereby, a biaxially oriented polyester film having an average surfaceroughness of 35 to 75 nm and good in “collapsing property” can beobtained by adding the two kinds of spherical silica fine particles.When the average surface roughness of the film is lower than 35 nm,sufficient slipperiness cannot be obtained and it is difficult to windthe film. Furthermore the pressing load is large and the defective shapeof capacitors tends to occur. When the average surface roughness islarger than 75 nm, it means that many particles having a large particlediameter are present, and the breakdown voltage is reduced.

It is preferable to reduce the average particle diameter from the filmthickness in order to stably produce the film. Further, the averageparticle diameter is preferably 90% or less based on the film thickness.

The method for producing the spherical silica particles is not limitedat all if the above conditions are satisfied; however, the sphericalsilica particles can be produced by hydrolyzing, for example, ethylorthosilicate [Si(OC₂H₅)₄], thereby preparing monodispersed spheres ofhydrous silica [Si(OH)₄], further carrying out dehydrating treatment ofthe monodispersed spheres of the hydrous silica and three-dimensionallygrowing the following silica bonds [see Nippon Kagaku Kaishi (Journal ofthe Chemical Society of Japan), No. 9, p. 1503, 1981].Si(OC₂H₅)₄+4H₂O→Si(OH)₄+4C₂H₅OH≡Si—OH+HO—Si≡→≡Si—O—Si≡+H₂O(≡Si—O—Si≡ . . . silica bond)

The amounts of the added spherical silica particles (A) and (B) in thepresent invention are preferably 0.03 to 1.5% by weight and 0.05 to 2%by weight, respectively. When the amount of the added spherical silicafine particles (A) is less than 0.03% by weight, the slipperiness oroperating efficiency of the film becomes insufficient. On the otherhand, when the amount exceeds 1.5% by weight, it is undesirable that thesurface hardness or space factor of the film is increased and thebreaking strength and breakdown voltage of the film are lowered. Whenthe amount of the spherical silica fine particles (B) is less than 0.05%by weight, the slipperiness or operating efficiency of the film becomesinsufficient. On the other hand, when the amount exceeds 2% by weight,it is undesirable that the breakage is increased during the slitting ofthe film. The total amount of the added spherical silica particles (A)and the spherical silica particles (B) is preferably 3% by weight orless for the reasons mentioned above.

The polyester containing the two kinds of spherical silica fineparticles can be produced by adding the spherical silica fine particles(preferably as a slurry in a glycol) into the reaction system in anoptional period during polymerizing reaction of the polymer, for examplein the transesterification in the case with a transesterification methodor in the polycondensation reaction. The spherical silica fine particlesare preferably added into the reaction system in the initial period ofthe polymerizing reaction, for example in a period until the intrinsicviscosity reaches about 0.3. Furthermore, besides the two kinds ofsilica fine particles, a lubricant having a smaller particle diameterthan that of the two kinds of spherical silica fine particles as a thirdcomponent may be added.

When the film of the present invention contains the spherical silicaparticles (A) and (B), the number of fly specks having an averagediameter exceeding 30 μm is preferably 10 fly specks/m² or less, morepreferably 8 fly specks/m² or less, especially preferably 5 flyspecks/m² or less. In order to ensure higher insulating performances asa dielectric of capacitors, the frequency of presence of fly speckshaving an average diameter exceeding 30 μm is preferably 10 flyspecks/m² or less.

<<Coarse Particles>>

The number of coarse particles having the maximum diameter exceeding 40μm present in a polyester film for capacitors of the present inventionis preferably smaller and preferably 10 coarse particles/m² or less,more preferably 8 coarse particles/m² or less, especially preferably 5coarse particles/m² or less. When the number of coarse particles havingthe maximum diameter exceeding 40 μm exceeds 10 coarse particles/m², itis undesirable that the frequency of parts where the thickness of theresin around the coarse particles is small is increased and theinsulating performances as a dielectric of the capacitors becomeinsufficient. For the same reasons, the number of coarse particleshaving the maximum diameter exceeding 35 μm is preferably smaller,preferably 10 coarse particles/m² or less, more preferably 8 coarseparticles/m² or less, especially preferably 5 coarse particles/m² orless.

<<Additives>>

Additives, for example a lubricant, a stabilizer or a flame retardantcan be contained in the polyester film for capacitors of the presentinvention. In order to impart slipperiness to the film, inert fineparticles such as inorganic particles, organic particles or cross-linkedpolymer particles in a small proportion are preferably contained for thepurpose of improving the runnability or handleability during production,processing and use of the film.

Examples of the inorganic particles include calcium carbonate, poroussilica, spherical silica, kaolin (platy aluminum silicate), talc,magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate,lithium phosphate, calcium phosphate, magnesium phosphate, aluminumoxide, silicon oxide, titanium oxide, zirconium oxide and lithiumfluoride.

Examples of the organic salt particles include calcium oxalate,terephthalates of calcium, barium, zinc, manganese and magnesium.

Examples of the cross-linked polymer particles include homopolymers orcopolymers of vinylic monomers of divinylbenzene, styrene, acrylic acid,methacrylic acid, acrylic acid or methacrylic acid. In addition, organicfine particles such as polytetrafluoroethylene, silicone resins,benzoguanamine resins, thermosetting epoxy resins, unsaturated polyesterresins, thermosetting urea resins, thermosetting phenol resins and thelike can be used.

The particle diameter of the inert fine particles is preferably 0.1 to 5μm expressed in terms of the average particle diameter. The averageparticle diameter is more preferably 0.15 μm or more and 4 μm or less,especially preferably 0.2 μm or more and 3 μm or less. When the averageparticle diameter is less than 0.1 μm, it is undesirable that improvingeffects on slipperiness are small, and that the addition concentration,therefore, must be extremely increased to increase the breakage in theproduction process for the film. When the average particle diameterexceeds 5 μm, it is undesirable that not only the breakage is increasedbut also pinholes due to falling of particles are increased.

In the present invention, the “average particle diameter” of the inertfine particles means the “equivalent spherical diameter” of particleslocated in a point of 50% by weight based on all the measured particles.The “equivalent spherical diameter” means the diameter of imaginaryspheres (ideal spheres) having the same volume as that of the particlesand can be determined by calculation from measurement of the particlesunder an electron microscope or by a usual settling method.

The total amount of the added inert fine particles is 0.05 to 3% byweight, more preferably 0.08 to 2.5% by weight, especially preferably0.1 to 2.0% by weight. When the amount is less than 0.05% by weight,improving effects on slipperiness are insufficient. When the amountexceeds 3% by weight, it is undesirable that breakage is increased inthe production process for the film.

The inert fine particles to be added to the film may be a singlecomponent selected from the exemplified particles or many componentscontaining two or three or more components. Particularly in thepolyester film for capacitors of the present invention, it is especiallypreferable to contain calcium carbonate and platy aluminum silicate inthe above exemplified p articles.

A nucleating agent, an antioxidant, a heat stabilizer, a lubricant, aflame retardant, an antistatic agent, a polysiloxane and the like can becompounded in the polyester film of the present invention according tothe use thereof.

The time for adding inert fine particles or other additives is notespecially limited if the time is in a stage untilpolyethylene-2,6-naphthalenedicarboxylate is formed into a film, and forexample the inert fine particles or other additives may be added in apolymerizing stage or during the film formation. From the viewpoint ofuniform dispersion, it is preferable to add the inert fine particles orother additives into ethylene glycol, add the inert fine particles orother additives at a high concentration during the polymerization andprovide master chips, which are diluted with chips without addition.

<<Method for Production>>

The polyester film for capacitors of the present invention can beproduced by a usual method for producing a biaxially stretched polyesterfilm.

For example, the polyester film for capacitors can be produced bymelting polyethylene-2,6-naphthalenedicarboxylate at a temperature notlower than the melting point thereof, extruding the melt from a die slitonto a casting drum at a controlled temperature in the vicinity of 60°C., bringing the extruded melt into close contact with the casting drum,cooling and solidifying the film, providing an unstretched film, thenbiaxially stretching the unstretched film in the machine direction andthe transverse direction, subsequently heat-setting the film, ifnecessary, subjecting the heat-set film to relaxing treatment in themachine direction and/or the transverse direction. In the presentinvention, it is necessary to adjust the thermal strain ratio RMD (150)of the film in the machine direction at 150° C. within the range of −1.5to 0.01%. For that purpose, the biaxial stretching is preferably carriedout by simultaneous biaxial stretching. The stretching of the film canbe conducted by using a known roll type longitudinal stretching machine,infrared heating longitudinal stretching machine, tenter clip typetransverse stretching machine, multistage stretching machine forcarrying out the stretching in divided plural stages, tubular stretchingmachine, oven type longitudinal stretching machine, simultaneous biaxialstretching machine and the like. Above all, the simultaneous biaxialstretching machine is preferable.

The method for producing the polyester film of the present inventionwill be explained in more detail hereinafter.

The production by the simultaneous biaxial stretching method will beinitially explained.

There have hitherto been a screw method for extending the clip intervalby carrying clips in screw channels and a pantograph method forextending the clip interval using the pantograph in the stretchingmechanism in the machine direction of the simultaneous biaxialstretching machine. The methods are not always ideal from the viewpointthat the film forming speed is low and a change in conditions such asstretch ratio is not easy; however, the methods can be used as astretching means in working the present invention.

On the other hand, a linear motor type simultaneous biaxial stretchingmachine has recently been developed and has attracted attention due to ahigh film formation speed and the like. The problems can be solved bythe linear motor type simultaneous biaxial stretching tenter at astroke. Thereby, the linear motor type simultaneous biaxial stretchingmachine is preferably used as a stretching means in working the presentinvention. The simultaneous biaxial stretching machine has advantages inthat flaws on the film surface are reduced because longitudinalstretching rollers as in sequential biaxial stretching are not used.Furthermore, planes of the benzene rings or naphthalene rings tend to beparallel to the film surface by the sequential biaxial stretching andthe refractive index nz in the thickness direction is decreased. Thetear propagation resistance is small and delamination is readily caused.On the other hand, the defects are improved by the simultaneous biaxialstretching. There is a structure capable of longitudinally relaxing thefilm in a heat-setting region in the simultaneous biaxial stretchingmachine and the thermal strain ratio in the machine direction at 150 to210° C. can be reduced. Since the preferable characteristics of thesimultaneous biaxial stretching machine are advantageous to productionof the polyester film for capacitors of the present invention, thepolyester film for capacitors of the present invention is preferablyproduced by the simultaneous biaxial stretching.

The simultaneous biaxial stretching described in the present inventionis stretching for simultaneously imparting orientation in the machinedirection and the transverse direction of the film and refers tooperations to transport the film while holding both ends of the filmwith clips and stretch the film in the machine direction and thetransverse direction using a simultaneous biaxial stretching machine.Further, the machine direction of the film is the longitudinal directionof the film and the transverse direction of the film is the widthdirection of the film. Naturally, there may be parts where stretching inthe machine direction and the transverse direction is simultaneouslycarried out with time and a method for solely prestretching the film inthe transverse direction or the machine direction and thensimultaneously stretching the film in the machine direction and thetransverse direction, a method for simultaneously biaxially stretchingthe film and then further solely stretching the film in the transversedirection or the machine direction and the like, therefore, are includedin the scope of the present invention.

In order to produce the polyester film for capacitors of the presentinvention, prescribed inert fine particles, i.e. necessary inert fineparticles in calcium carbonate particles, platy ammonium silicateparticles, porous silica and spherical silica and further, if necessary,other inert fine particles are contained in the polymer and then meltextruded at a usual extruding temperature, i.e. a temperature not lowerthan the melting temperature (hereinafter referred to Tm) and not higherthan (Tm+70)° C., and the melt extruded filmlike melt is subsequentlyquenched on the surface of a rotating cooling drum to provide anunstretched film having an intrinsic viscosity of, for example 0.40 to0.90 dl/g.

Preferable conditions for obtaining the unstretched film by usingpolyethylene-2,6-naphthalenedicarboxylate polymer are 170° C. and about6 hours for drying the polymer, an extruding temperature of the polymerin the vicinity of 300° C. and the surface temperature of a cooling drumof about 60° C. The ratio of the thickness at the end to the centralpart-of the unstretched film (thickness at the end/thickness in thecentral part) is preferably 1 or more and 10 or less, more preferably 1or more and less than 5, far more preferably 1 or more and less than 3.When the ratio of the thickness is less than 1 or exceeds 10, it isundesirable that the film breakage or film slip off the clip at theproduction occurs.

The resulting unstretched film is led to a simultaneous biaxialstretching machine by holding both ends of the unstretched film withclips, heated in a preheating zone [glass transition point temperatureTg of the polyester −40] to (Tg+50)° C. in the preheating zone and thensimultaneously biaxially stretched at (Tg−10) to (Tg+70)° C. at an arearatio of 10 to 40 times (longitudinal ratio of 2 to 6 times) in onestage or many stages of two or more stages.

The stretched film, if necessary, may then be simultaneously biaxiallystretched at a temperature within the range of (melting point Tm of thepolyester −120) to (Tm−10)° C. at an area ratio of 2 to 5 times in onestage or many stages of two or more stages, subsequently heat-set at atemperature within the range of (Tm−70) to (Tm)° C. and, if necessary,subjected to relaxing treatment within a temperature range of preferably100 to 245° C. in the machine direction and the transverse direction ata ratio within the range of 1 to 10% for each direction in a coolingprocess from heat-setting while, if necessary, carrying out theheat-setting.

When the unstretched film of polyethylene-2,6-naphthalenedicarboxylateis stretched, preferable conditions are a preheating temperature ofabout 140° C., a stretching temperature of about 145° C. and aheat-setting temperature of about 240° C. The film is subjected torelaxing treatment in the machine direction and the transverse directionwhile, if necessary, carrying out the heat-setting, then cooled to roomtemperature and wound to afford the objective simultaneous biaxiallystretched polyester film.

In the present invention, the surface of the polyester film ispreferably coated with a coating agent in a step before or after thesimultaneous biaxial stretching in order to impart surfacecharacteristics of the film, for example easy adhesion, slipperiness,mold release properties and antistatic properties.

Furthermore, the polyester film for capacitors of the present inventioncan be produced by usual sequential biaxial stretching. The unstretchedpolyethylene-2,6-naphthalenedicarboxylate film obtained by a knownmethod is stretched at 120 to 180° C., more preferably 125 to 170° C.,especially preferably 130 to 160° C. in the machine direction at 3.0 to5.0 times, more preferably 3.3 to 4.6 times by a roll type longitudinalstretching machine. An infrared heating type longitudinal stretchingmachine may be used; however, a roll type longitudinal stretchingmachine is preferable that it is advantageous to uniform heating of thewhole film especially when a thin film is stretched. The stretching ispreferably carried out in plural divided stages in order to reasonablystretch the film by longitudinal stretching. After the longitudinalstretching, the resulting film is then stretched at 120 to 180° C., morepreferably 125 to 170° C., especially preferably 130 to 160° C. in thetransverse direction at 3.0 to 4.5 times, more preferably 3.5 to 4.3times, heat-treated at preferably 195 to 250° C., more preferably 205 to245° C. for 0.3 to 50 seconds and then subjected to heat relaxingtreatment in the machine direction and/or the transverse direction at arelaxing ratio within the range of 0.5 to 15% in a stenter. Thereby, thepolyester film for capacitors of the present invention can be obtained.Further, multistage stretching in which the stretching in the transversedirection is divided in plural stages may be used.

Furthermore, in order to adjust the number of fly specks having anaverage diameter exceeding 60 μm, 55 μm and 30 μm within the aboverange, the molten polymer is preferably filtered out through a nonwovenfabric type filter which comprises stainless steel fine wires having awire diameter of 15 μm or less and has an average sieve opening of 10 to40 μm, preferably 15 to 35 μm, more preferably 10 to 35 μm, especiallypreferably 15 to 30 μm as a filter during the film formation. When thesieve opening of the filter exceeds 40 μm, there are no effects onreduction of coarse particles in the molten polymer. On the other hand,when the sieve opening is less than 10 μm, the clogging of the filtertends to occur and the industrial practical use is difficult. Inaddition, there are filters of a netlike structure or using a sinteredmetal or the like; however, there are problems that the life is shorterthan that of the nonwoven fabric type filter and the filtrationefficiency is somewhat inferior and the like. Furthermore, in order tomore effectively adjust the number of fly specks having a large sizewithin the above range, the lubricant itself is preferably filteredthrough the filter having a prescribed sieve opening and then added intothe polymer.

The polyester film of the present invention is used as a dielectric offilm capacitors and is preferably applied to film capacitors used inplaces at a higher service environmental temperature because the glasstransition temperature of the dielectric film is higher than that of theconventional polyester (polyethylene terephthalate) film capacitors. Inparticular, the polyester film is preferably used in rectifier circuitsmore adjacent to heat sources than conventional rectifier circuits dueto miniaturization of electrical and electronic equipment, circuits ofelectrical instrumentation parts installed around engines or in vehiclesas electrical instrumentation parts of automobiles and the like becausethe service environmental temperature becomes high temperatures. Thepolyester film is preferably used as a film for capacitors requiringwithstand voltage characteristics under high voltages or stabilizationof capacity under a high frequency such as transformer circuits, currentconverting circuits and the like of hybrid cars, electric cars,electronic exchanges and the like.

EXAMPLES

The present invention will be explained in more detail with Examples.Various physical properties or methods for measuring the characteristicsand definitions in the present invention are as follows.

(1) Calculation of Amounts of Components (Molar Ratio of the PrincipalComponent and Molar Ratios of Copolymerization Component) ofpolyethylene-2,6-naphthalenedicarboxylate

A film sample was dissolved in a measuring solvent (CDCl₃:CF₃COOD=1:1),and ¹H-NMR measurement was then carried out to calculate the amounts ofthe components using integral ratios of the resulting respectivesignals.

(2) Thermal Strain Ratio

Sampling into a strip form of 4 mm in width long in the film machinedirection (MD) was carried out and the sample was heated up from roomtemperature at a heat-up rate of 5° C./min in a state of an appliedconstant load of 2.0 MPa in the machine direction by using a measurementmodule TMA/SS120C model manufactured by Seiko Instruments Inc. and athermal analysis system SSC/5200H model manufactured by the same companyas a data analyzer. The length of the film was measured at 35° C., 150°C. and 210° C. and the thermal strain ratio RMD in the machine directionwas calculated by the calculating formula. The thermal strain ratio RTDin the transverse direction (TD) was obtained according to theprocedures described above except that a sample long in the transversedirection (TD) was cut out.

(3) Fly Specks

The film surface was magnified 50 times by illumination with polarizedlight transmission using a universal measuring projector and fly speckspresent in an area of 1 m² were observed and marked. The respective flyspecks were observed under an optical microscope to determine theaverage diameter, i.e. (major axis+minor axis)/2 including nuclei of thefly specks and voids formed around the nuclei. The numbers of the flyspecks having an average diameter exceeding 60 μm, 55 μm and 30 μm werecounted, respectively.

(4) Coarse Particles

The film surface was magnified 50 times by illumination withtransmission using a universal measuring projector and a measuring areaof 1 m² was observed to count the numbers of particles having themaximum diameter exceeding 35 μm and the number of particles having themaximum diameter exceeding 40 μm in particles present in the film.

(5) 5% Strain Strength (F5 Value)

A film cut into a sample width of 10 mm and a length of 150 mm andpulled at a length zipping the sample both-side (i.e. marking length ofsample) of 100 mm, a tensile speed of 100 mm/min and a chart speed of500 mm/min using an Instron type universal tensile tester. Measurementin both the the directions of MD and TD was carried out and the strengthat 5% elongation was read from the resultant load-elongation curve anddivided by the original cross-sectional area to obtain the 5% strainstrength (N/mm²). The F5 value in the MD direction was divided by the F5value in the TD direction to calculate the machine direction/ransversedirection ratio.

(6) Surface Roughness (Center Line Average Height: Ra, Ten-Point AverageRoughness: Rz)

The center line average height (Ra) was a value defined by JIS B 0601.In the present invention, the protrusion profile on the film surface wasmeasured under conditions of a measuring length (Lx) of 1 mm, a samplingpitch of 2 μm, a cutoff of 0.25 mm, a magnification of 10000 times inthe thickness direction, a magnification in the planar direction of 200times and a number of scanning lines of 100 (Ly=0.2 mm) withsemiconductor laser at a wavelength of 780 nm and an optical stylushaving a beam diameter of 1.6 μm using a non-contact typethree-dimensional roughness measuring instrument (T-30HK manufactured byKosaka Laboratory Ltd.). Thereby, the surface roughnesses (Ra and Rz)were calculated.

(7) Density

The density was measured by a sink-and-float method with a densitygradient tube using an aqueous solution of calcium nitrate at 25° C.

(8) Film Thickness and Dispersion of Thickness

Fifty samples having a size of length 10 cm×width 10 cm were collectedfrom optional sites of a formed film at an interval of 10 cm or more inboth the machine direction and the transverse direction. The thicknesses(μm) T1, T2 . . . T50 of the respective samples were calculated from thewidth (cm), length (cm), weight (g) and density (g/cm³) by the followingformulae to obtain the average thickness Tav of the 50 samples. Thereby,the film thickness was determined. The difference between the maximumthickness Tmax and the minimum thickness Tmin in the 50 samples wasobtained to calculate the ratio to the average thickness Tav by thefollowing formulae to provide the dispersion of the thickness.

-   Thickness (μm)=[weight/(width×length×density)×10000-   Average thickness Tav (μm)=(T1+T1+ . . . +T50)/50-   Dispersion of thickness (%)=[(Tmax−Tmin)/average thickness Tav]×100    (9) Average Particle Diameter of Inert Fine Particles

The average particle diameter was measured by using CP-50 modelCentrifugal Particle Size Analyzer) manufactured by Shimadzu Corp., andthe particle diameter corresponding to 50% by weight was read from acumulative curve of particles having the respective particle diametersand the amount of the presence thereof calculated on the basis of theresulting centrifugal settling curve [see “Technique of Particle SizeMeasurement”, published by THE NIKKAN KOGYO SHINBUN LTD., 1975, pp.242-247].

(10) Particle Diameter Ratio of Inert Fine Particles

a) In the Case of Particles in Film

A small piece of a sample film was fixed on a stage for a scanning typeelectron microscope and ion etching treatment of the film surface wascarried out under the following conditions using a sputtering apparatus(JFC-1100 model ion sputtering apparatus) manufactured by JEOL Ltd. Theconditions were as follows. The sample was placed in a bell jar, and theion etching treatment was carried out in a vacuum state of about 10⁻³Torr under a voltage of 0.25 kV at a current of 12.5 mA for about 10minutes. The film surface was then subjected to gold sputtering with thesame apparatus and observed under a scanning type electron microscope ata magnification of 10000 to 30000 times, and the major axis and minoraxis of at least 200 particles were measured with Luzex 500 manufacturedby Nippon Regulator Co., Ltd. The total of the measured values weredivided by the number of measurement for each of the major axis andminor axis to determine the average major axis and average minor axis.The particle diameter ratio was obtained from the average major axis andthe average minor axis.

b) In the Case From Powder

Particle powder was scattered on the stage of an electron microscopewith the least possible overlapping of the individual particles, and thesurface of a gold film vapor deposition layer was formed on the surfacewith a gold sputtering apparatus to a thickness of 200 to 300 angstromsand observed at a magnification of 10000 to 30000 times under a scanningtype electron microscope. The major axis and minor axis of at least 100particles (measured values in Table 2) or at least 200 particles(measured values in Table 3) were measured with Luzex 500 manufacturedby Nippon Regulator Co., Ltd., and the average value of the respectivevalues were calculated in the same manner as in the above a) todetermine the particle diameter ratio.

(11) Pore Volume

The pore volume was measured by a nitrogen adsorption and desorptionmethod and calculated by a BET formula.

(12) Film-Forming Properties of Film

The film-formed state of the film when the biaxially stretched film wascontinuously formed for 24 hours was observed and evaluated according tothe following criteria.

-   ⊚: The frequency of breakage is 0/24 hours and extremely stable film    formation can be carried out.-   ◯: The frequency of breakage is 1 to 3 times/24 hours and stable    film formation can be carried out.-   X: The frequency of breakage is 4 times or more/24 hours and film    formation is unstable.    (13) Take-Up Form of Film Roll

The take-up form of a film roll after winding into a roll at a width of550 mm and a length of 10000 m was respectively observed during the filmformation and the slitting of the film and after vapor deposition incapacitor processing and evaluated according to the following criteria.

-   ⊚: No surface crease, wander and the like by observation with naked    eyes during the film formation and the slitting of the film and    after vapor deposition and good take-up form.-   X: Surface creases, wander and the like are confirmed by observation    with naked eyes during the film formation and the slitting of the    film and/or vapor deposition and the take-up form is poor.    (14) Insulation Resistance

Two hundred capacitor samples of 0.1 μF were placed under an atmosphereat 23° C. and 65% RH and insulation resistance was measured as a valueof 1 minute under an applied voltage of 500 V with a superinsulationresistance meter 4329A model manufactured by Yokogawa Hewlett PackardLtd. Capacitor samples having an insulation resistance of less than 5000MΩ were taken as defectives and judged according to the followingcriteria. In the invention, ⊚ and Δ were regarded as passing theinspection.

-   ⊚: Less than 4 defectives.-   Δ: Four or more and less than 10 defectives-   X: Ten or more defectives.    (15) Breakdown Voltage (BDV)

The breakdown voltage was measured according to the method defined byJIS C 2318, and the minimum of n=200 was the breakdown voltage (BDV). Inthe present invention, the breakdown voltage is preferably 220 V/μm ormore, especially preferably 240 V/μm or more.

Example 1

Transesterification was carried out by using 100 parts of dimethyl2,6-naphthalenedicarboxylate, 60 parts of ethylene glycol and 0.03 partof manganese acetate tetrahydrate as a transesterification catalyst andadding calcium carbonate particles having an average particle diameterof 1.0 μm and kaolin clay particles (platy aluminum silicate particles)having an average particle diameter of 0.7 μm so as to contain 0.45% byweight of the calcium carbonate particles and 0.30% by weight of thekaolin clay particles according to a conventional method and 0.023 partof trimethyl phosphate was then added to substantially complete thetransesterification.

To the reaction mixture, was then added 0.024 part of antimony trioxide.The polymerizing reaction was subsequently carried out at a hightemperature and high decompression according to a conventional method toprovide a polyethylene-2,6-naphthalenedicarboxylate having an intrinsicviscosity of 0.62 dl/g (PEN Tg=121° C.). The resulting PEN polymer wasthen dried at 170° C. for 5 hours, subsequently fed to an extruder,melted at a melting temperature of 295° C., filtered through a nonwovenfabric type filter comprising stainless steel fine wires having a wirediameter of 14 μm and an average sieve opening of 30μ, extruded througha die slit and then cooled and solidified on a casting drum to preparean unstretched film.

The obtained unstretched film was introduced into a simultaneous biaxialstretching machine, preheated at 140° C. and then simultaneouslystretched at 4.5 times in the machine direction and 4.2 times in thetransverse direction at 145° C. while holding the film with clips. Thestretched film was then respectively heat-set at 200° C., 230° C. and235° C. in the first, second and third heat-setting zone for 2 secondseach to impart 3% longitudinal relaxation and afford a biaxiallyoriented film having a thickness of 3.0 μm. Table 1 shows physicalproperties and evaluation results of the biaxially oriented film.

The resulting polyester film was treated at 70° C. for 24 hours afterthe passage of 1 week from the film formation and one surface was thensubjected to vacuum evaporation of aluminum so as to afford a surfaceresistance value of 2 Ω/□. In the process, the vapor deposition wascarried out so as to provide a stripe form having marginal parts runningin the longitudinal direction (repetition of a width of 58 mm in thevapor deposition part and a width of 2 mm of the marginal parts). Theresulting vapor deposited film was then subjected to voltage treatmentfor applying a direct current 800 V to the full width of the film andblades were inserted into the center of the respective vapor depositedparts and the center of the respective marginal parts to carry outslitting. Thereby, a tapelike wound reel having the full width of 30 mmwith a margin of 1 mm in the left or right was obtained.

Each sheet of the resulting reel of the left margin and right margin wassuperimposed and wound to afford a piece of capacitor having anelectrostatic capacity of 0.1 μF. The piece of capacitor was pressed ata temperature of 150° C. under a pressure of 196 MPa for 5 minutes.Metallicon was thermally sprayed on both end faces thereof to provideexternal electrodes. Lead wires were then welded to the metallicon andexternally packaged and cured with an epoxy resin to provide a rolledtype capacitor. Table 1 shows the evaluation results as the capacitor.The capacitor had a breakdown voltage as high as 270 V/μm and wasexcellent in insulation resistance characteristics and especiallyexcellent in film-forming properties.

Example 2

The amount of the inert fine particles to be added was adjusted to thatmentioned in Table 1 and an unstretched film was prepared in the samemanner as in Example 1. The resulting film was stretched at 135° C. inthe machine direction (longitudinal direction) at 1.8 times, thensequentially stretched at 145° C. in the machine direction at 2 times(longitudinal stretching ratio of 3.6 times) and stretched at 140° C. inthe transverse direction (width direction) at 3.6 times and subsequentlyheat-set at 232° C. for 3 seconds to afford a biaxially oriented filmhaving a thickness of 3.0 μm, which was wound into a roll. The filmsimple material was evaluated and a piece of capacitor was prepared inthe same manner as in Example 1 and evaluated. Table 1 shows theresults. The capacitor had a breakdown voltage as high as 260 V/μm andexcellent in insulation resistance characteristics.

Reference Example 1

Procedures were carried out in the same manner as in Example 1, exceptthat the average sieve opening of the melt filtering filter was adjustedto 50 μm. Thereby, a film was formed to prepare a piece of capacitor,which was evaluated. Table 1 shows the results. The capacitor had manyfly specks and a low breakdown voltage.

Comparative Example 1

A film was formed in the same manner as in Example 1, except that theheat-setting temperature was 200° C. in each zone without impartinglongitudinal relaxing. A piece of capacitor was prepared and evaluated.Table 1 shows the results. The piece of capacitor has a large thermalstrain ratio in the machine direction at 150° C. on the negative sideand defective shapes of capacitor frequently occurred.

Comparative Example 2

The stretching conditions were changed into those mentioned in Table 1in Example 2. Table 1 shows the evaluation results of the film and pieceof capacitor. The resulting piece of capacitor has a large thermalstrain ratio in the machine direction at 150° C. on the positive sideand a low insulation resistance.

TABLE 1 Reference Comparative Comparative Example 1 Example 2 Example 1Example 1 Example 2 Polyester Polyethylene-2,6-naphthalenedicarboxylateKind of lubricant Lubricant 1 Calcium Calcium Calcium Calcium Calciumcarbonate carbonate carbonate carbonate carbonate Lubricant 2 Kaolinclay Kaolin clay Kaolin clay Kaolin clay Kaolin clay Average particlediameter Lubricant 1 1.0 1.0 1.0 1.0 1.0 (μm) Lubricant 2 0.7 0.7 0.70.7 0.7 Addition concentration Lubricant 1 0.45 0.45 0.45 0.45 0.45 (%by weight) Lubricant 2 0.30 0.25 0.30 0.30 0.25 Stretching methodSimultaneous Sequential Simultaneous Simultaneous Sequential biaxialbiaxial biaxial biaxial biaxial Stretching ratio Machine direction 4.53.6 4.5 4.5 2.3 Transverse direction 4.2 3.6 4.2 4.2 3.3 Heat-settingtemperature (°C.) 235 232 235 200 245 Thickness (μm) 3.0 3.0 3.0 3.0 3.0Dispersion of thickness (%) 9 12 9 8 35 Fly specks Average diameter >60μm 5 5 36 5 5 (fly specks/m2) Coarse particles Maximum diameter >35 μm 44 47 4 4 (coarse particles/m2) 5% strain strength (MPa) MD 156 138 154160 115 TD 140 136 138 142 130 Machine direction/transverse directionratio 1.11 1.01 1.12 1.13 0.88 of 5% strain strength (MD/TD) Surfaceroughness (nm) Ra 64 62 68 63 61 Intrinsic viscosity (dl/g) 0.54 0.540.54 0.54 0.54 0.54 Density (g/cm3) 1.353 1.353 1.353 1.353 1.351 1.356Thermal strain ratio (%) RMD (150) −1.0 −1.2 −1.0 −1.6 0.5 RTD (150)−0.7 −0.8 − 0.7 − 0.8 0.1 RMD (210) −2.3 −2.5 −2.3 −3.7 0.1 RTD (210)−2.0 −2.1 −2.0 −2.8 −0.8 Film-forming property of film ⊚ ◯ X ⊚ ◯ Take-upform of film roll ⊚ ⊚ ⊚ ⊚ X Breakdown voltage (V/μm) 270 260 150 270 220Insulation resistance of a piece of capacitor ◯ ◯ Δ ◯ X

Example 3

Transesterification was carried out by using 100 parts of dimethyl2,6-naphthalenedicarboxylate and 60 parts of ethylene glycol and 0.03part of manganese acetate tetrahydrate as a transesterification catalystand adding porous silica particles having an average particle diameterof 2.0 μm and a pore volume of 1.2 ml/g and spherical silica particleshaving an average particle diameter of 0.6 μm so as contain 0.35% byweight of the porous silica particles and 0.30% by weight of thespherical silica particles according to a conventional method and 0.023part of trimethyl phosphate was then added to substantially complete thetransesterification.

To the reaction mixture, was then added 0.024 part of antimony trioxide.The polymerizing reaction was subsequently carried out at a hightemperature and high decompression according to a conventional method toprovide a polyethylene-2,6-naphthalenedicarboxylate having an intrinsicviscosity of 0.62 dl/g (PEN Tg=121° C.). The resulting PEN polymer wasthen dried at 170° C. for 5 hours, subsequently fed to an extruder,melted at a melting temperature of 295° C., filtered through a nonwovenfabric type filter comprising stainless steel fine wires having a wirediameter of 14 μm and an average sieve opening of 30μ, extruded througha die slit and then cooled and solidified on a casting drum to preparean unstretched film.

The obtained unstretched film was introduced into a simultaneous biaxialstretching machine, preheated at 140° C. and then simultaneouslystretched at 4.4 times in the machine direction and 4.1 times in thetransverse direction at 145° C. while holding the film with clips. Thestretched film was then respectively heat-set at 200° C., 230° C. and235° C. in the first, second and third heat-setting zone for 2 secondseach to impart 3% longitudinal relaxation and afford a biaxiallyoriented film having a thickness of 2.0 μm. Table 2 shows physicalproperties and evaluation results of the biaxially oriented film.

The resulting polyester film was treated at 70° C. for 24 hours afterthe passage of 1 week from the film formation and one surface was thensubjected to vacuum evaporation of aluminum so as to afford a surfaceresistance value of 2 Ω/□. In the process, the vapor deposition wascarried out so as to provide a stripe form having marginal parts runningin the longitudinal direction (repetition of a width of 58 mm in thevapor deposition part and a width of 2 mm of the marginal parts). Theresulting vapor deposited film was then subjected to voltage treatmentfor applying a direct current 800 V to the full width of the film andblades were inserted into the center of the respective vapor depositedparts and the center of the respective marginal parts to carry outslitting. Thereby, a tapelike wound reel having the full width of 30 mmwith a margin of 1 mm in the left or right was obtained.

Each sheet of the resulting reel of the left margin and right margin wassuperimposed and wound to afford a piece of capacitor having anelectrostatic capacity of 0.1 μF. The piece of capacitor was pressed ata temperature of 150° C. under a pressure of 196 MPa for 5 minutes.Metallicon was thermally sprayed on both end faces thereof to provideexternal electrodes. Lead wires were then welded to the metallicon andexternally packaged and cured with an epoxy resin to provide a rolledtype capacitor. Table 1 shows the evaluation results as the capacitor.The capacitor had a breakdown voltage as high as 270 V/μm and wasexcellent in insulation resistance characteristics.

Example 4

The average particle diameter of the inert fine particles to be addedwas changed to, that mentioned in Table 2 and an unstretched film wasprepared in the same manner as in Example 3. The resulting film wasstretched at 1.7 times in the machine direction (longitudinal direction)at 135° C., then sequentially biaxially stretched at 145° C. in themachine direction at 2 times (longitudinal stretching ratio of 3.4times) and stretched at 140° C. in the transverse direction (widthdirection) at 3.5 times and subsequently heat-set at 232° C. for 3seconds to afford a biaxially oriented film having a thickness of 2.0μm, which was wound into a roll. The film simple material was evaluatedand a piece of capacitor was prepared in the same manner as in Example 1and evaluated. Table 1 shows the results. The capacitor had a breakdownvoltage as high as 270 V/μm and was excellent in insulation resistancecharacteristics.

Reference Example 2

Procedures were carried out in the same manner as in Example 3, exceptthat the average sieve opening of the melt filtering filter was adjustedto 50 μm. Thereby, a film was formed to prepare a piece of capacitor,which was evaluated. Table 2 shows the results. The capacitor had manyfly specks and a low breakdown voltage.

Comparative Example 3

Procedures were carried out in the same manner as in Example 3, exceptthat the lubricant composition was adjusted to that shown in Table 2 andthe heat-setting temperature in each zone was adjusted to 190° C.without imparting longitudinal relaxation. Thereby, a film was 15 formedto prepare a piece of capacitor, which was evaluated. Table 2 shows theresults. The capacitor had a large thermal strain ratio in the machinedirection at 150° C. on the negative side and defective shapesfrequently occurred.

Comparative Example 4

Stretching conditions were changed to those mentioned in Table 2 inExample 4. Table 1 shows evaluation results of the resulting film andthe piece of capacitor. The obtained piece of capacitor had a largethermal strain ratio in the machine direction at 150° C. on the positiveside and a low insulation resistance.

TABLE 2 Reference Comparative Comparative Example 3 Example 4 Example 2Example 3 Example 4 Polyester Polyethylene-2,6-naphthalenedicarboxylateKind of lubricant Lubricant 1 Porous silica Porous silica Porous silicaPorous silica Porous silica Lubricant 2 Spherical silica Sphericalsilica Spherical silica — Spherical silica Average particle diameterLubricant 1 2.00 1.80 2.00 0.50 1.80 (μm) Lubricant 2 0.60 0.60 0.60 —0.60 Addition concentration Lubricant 1 0.35 0.35 0.35 0.20 0.35 (% byweight) Lubricant 2 0.30 0.30 0.30 — 0.30 Particle diameter ratio oflubricant 2 1.1 1.1 1.1 1.1 1.1 Stretching method SimultaneousSequential Simultaneous Simultaneous Sequential biaxial biaxial biaxialbiaxial biaxial Stretching ratio Machine direction 4.4 3.4 4.4 4.4 2.5Transverse direction 4.1 3.5 4.1 4.1 3.4 Heat-setting temperature (°C.)235 232 235 190 240 Thickness (μm) 2.0 2.0 2.0 2.0 2.0 Dispersion ofthickness (%) 10 13 10 8 31 Number of coarse particles Maximum 3 2 19 02 diameter >40 μm (coarse particles/m2) Fly specks Average diameter > 54 34 0 4 55 μm (fly specks/m2) 5% strain strength (MPa) MD 150 134 148158 113 TD 137 136 138 140 133 Machine direction/transverse directionratio 1.09 0.99 1.07 1.13 0.85 of 5% strain strength (MD/TD) Surfaceroughness (nm) Ra 72 70 73 13 70 Rz 1610 1660 1630 380 1650 Intrinsicviscosity (dl/g) 0.55 0.55 0.55 0.55 0.55 Density (g/cm3) 1.353 1.3531.353 1.349 1.356 Thermal strain ratio (%) RMD (150) −0.8 −1.1 − 0.8−2.0 0.5 RTD (150) −0.6 −0.8 −0.6 −1.3 − 0.1 RMD (210) −2.1 −2.3 −2.1−4.1 0.0 RTD (210) −2.0 −2.0 −2.0 −3.2 −1.2 Film-forming property offilm ⊚ ◯ X ⊚ ◯ Take-up form of film roll ⊚ ⊚ ⊚ X X Breakdown voltage(V/μm) 270 270 170 300 220 Insulation resistance of a piece of capacitor◯ ◯ Δ Δ X

Example 5

Transesterification was carried out by using 100 parts of dimethyl2,6-naphthalenedicarboxylate, 60 parts of ethylene glycol and 0.03 partof manganese acetate tetrahydrate as a transesterification catalyst andadding spherical silica particles (A) having a particle diameter ratioof 1.1 and an average particle diameter of 1.0 μm and spherical silicaparticles (B) having a particle diameter ratio of 1.1 and an averageparticle diameter of 0.5 μm so as to contain 0.40% by weight of thespherical silica particles (A) and 0.30% by weight of the sphericalsilica particles (B) according to a conventional method and 0.023 partof trimethyl phosphate was then added to substantially complete thetransesterification. The particle diameter ratio of the spherical silicaparticles used is each 1.1 in any of Examples and Comparative Examples.

To the reaction mixture, was then added 0.024 part of antimony trioxide.The polymerizing reaction was subsequently carried out at a hightemperature and high decompression according to a conventional method toprovide a polyethylene-2,6-naphthalenedicarboxylate having an intrinsicviscosity of 0.62 dl/g (PEN Tg=121° C.). The resulting PEN polymer wasthen dried at 170° C. for 5 hours, subsequently fed to an extruder,melted at a melting temperature of 295° C., filtered through a nonwovenfabric type filter comprising stainless steel fine wires having a wirediameter of 14 μm and an average sieve opening of 30μ, extruded througha die slit and then cooled and solidified on a casting drum to preparean unstretched film.

The obtained unstretched film was introduced into a simultaneous biaxialstretching machine, preheated at 140° C. and then simultaneouslystretched at 4.2 times in the machine direction and 4.0 times in thetransverse direction at 145° C. while holding the film with clips. Thestretched film was then respectively heat-set at 210° C., 230° C. and237° C. in the first, second and third heat-setting zone for 2 secondseach to impart 3% longitudinal relaxation and afford a biaxiallyoriented film having a thickness of 4.0 μm. Table 3 shows physicalproperties and evaluation results of the biaxially oriented film.

The resulting polyester film was treated at 70° C. for 24 hours afterthe passage of 1 week from the film formation and one surface was thensubjected to vacuum evaporation of aluminum so as to afford a surfaceresistance value of 2 Ω/□. In the process, the vapor deposition wascarried out so as to provide a stripe form having marginal parts runningin the longitudinal direction (repetition of a width of 58 mm in thevapor deposition part and a width of 2 mm of the marginal parts). Theresulting vapor deposited film was then subjected to voltage treatmentfor applying a direct current 800 V to the full width of the film andblades were inserted into the center of the respective vapor depositedparts and the center of the respective marginal parts to carry outslitting. Thereby, a tapelike wound reel having the full width of 30 mmwith a margin of 1 mm in the left or right was obtained.

Each sheet of the resulting reel of the left margin and right margin wassuperimposed and wound to afford a piece of capacitor of anelectrostatic capacity of 0.1 μF. The piece of capacitor was pressed ata temperature of 150° C. under a pressure of 196 MPa for 5 minutes.Metallicon was thermally sprayed on both end faces thereof to provideexternal electrodes. Lead wires were then welded to the metallicon,externally packaged and cured with an epoxy resin to provide a rolledtype capacitor. Table 3 shows the evaluation results as the capacitor.The capacitor had a breakdown voltage as high as 290 V/μm and wasexcellent in insulation resistance characteristics and especiallyexcellent in film-forming properties.

Example 6

The average particle diameter and addition concentration of the inertfine particles to be added as a lubricant was changed to those mentionedin Table 3 and an unstretched film was prepared in the same manner as inExample 5. The resulting film was stretched at 135° C. in the machinedirection (longitudinal direction) at 1.9 times, then sequentiallybiaxially stretched at 145° C. in the machine direction at 2 times(longitudinal stretching ratio of 3.8 times) and stretched at 145° C. inthe transverse direction (width direction) at 3.6 times and subsequentlyheat-set at 234° C. for 3 seconds to afford a biaxially oriented filmhaving a thickness of 4.0 μm, which was wound into a roll. The filmsimple material was evaluated and a piece of capacitor was prepared inthe same manner as in Example 1 and evaluated. Table 1 shows theresults. The capacitor had a breakdown voltage as high as 290 V/μm andwas excellent in insulation resistance characteristics.

Reference Example 3

Procedures were carried out in the same manner as in Example 5, exceptthat the average particle diameter and the addition concentration of theinert fine particles to be added as a lubricant were changed to thosementioned in Table 3 and a filter having an average sieve opening of 50μm was used as the melt filtering filter the melt filtering filter.Thereby, a film was formed to prepare a piece of capacitor, which wasevaluated. Table 1 shows the results. In this case, no piece ofcapacitor satisfying the number of fly specks which was the requirementof the present invention was obtained, and merely the breakdown voltageas low as, 190 V/μm was obtained.

Comparative Example 5

Procedures were carried out in the same manner as in Example 5, exceptthat the lubricant composition was adjusted to that shown in Table 3 andthe heat-setting temperature in each zone was adjusted to 195° C.without imparting longitudinal relaxation. Thereby, a film was formed toprepare a piece of capacitor, which was evaluated. Table 3 shows theresults. The capacitor had a large thermal strain ratio in the machinedirection at 150° C. on the negative side and the take-up form of thefilm roll was poor. Defective shapes of the capacitor frequentlyoccurred.

Comparative Example 6

Procedures were carried out in the same manner as in Example 6, exceptthat the thermal strain ratio of the film was changed by changing thestretching conditions. Thereby, a film was formed to prepare a piece ofcapacitor, which was evaluated. Table 3 shows the results. The piece ofcapacitor had a large thermal strain in the machine direction at 150° C.on the positive side and the take-up form of the film roll was poor. Theinsulation resistance characteristics were inferior.

TABLE 3 Reference Comparative Comparative Example 5 Example 6 Example 3Example 5 Example 6 Polyester Polyethylene-2,6-naphthalenedicarboxylateKind of lubricant Lubricant 1 Spherical silica Spherical silicaSpherical silica Spherical silica Spherical silica Lubricant 2 Sphericalsilica Spherical silica Spherical silica — Spherical silica Averageparticle diameter Lubricant 1 1.0 2.0 3.5 0.4 2.0 (μm) Lubricant 2 0.50.2 0.6 — 0.2 Addition concentration Lubricant 1 0.4 0.35 0.35 0.1 0.35(% by weight) Lubricant 2 0.30 0.25 0.25 — 0.25 Stretching methodSimultaneous Sequential Simultaneous Simultaneous Sequential biaxialbiaxial biaxial biaxial biaxial Stretching ratio Machine direction 4.23.8 4.2 4.2 2.8 Transverse direction 4.0 3.6 4.0 4.0 3.2 Heat-settingtemperature (° C.) 237 234 237 195 238 Thickness (μm) 4.0 4.0 4.0 4.04.0 Dispersion of thickness (%) 10 12 10 8 29 Fly specks Averagediameter >30 μm 2 3 14 0 3 (fly specks/m2) 5% strain strength (MPa) MD145 140 143 151 124 TD 135 133 135 138 126 Machine direction/transversedirection ratio 1.07 1.05 1.06 1.09 0.98 of 5% strain strength (MD/TD)Surface roughness (nm) Ra 58 64 77 9 62 Rz 1200 1450 1980 270 1400Intrinsic viscosity (dl/g) 0.52 0.52 0.52 0.52 0.52 Density (g/cm3)1.356 1.354 1.353 1.351 1.354 Thermal strain ratio (%) RMD (150) −0.6−1.3 −0.6 −1.8 0.3 RTD (150) −0.6 −0.7 −0.6 −1.3 0.0 RMD (210) −1.9 −2.9 − 1.9 −3.8 −0.1 RTD (210) −1.6 −2.0 −1.6 −3.0 −1.5 Film-formingproperty of film ⊚ ◯ X ⊚ ◯ Take-up form of film roll ⊚ ⊚ ⊚ X X Breakdownvoltage (V/μm) 290 290 190 300 230 Insulation resistance of a piece ofcapacitor ◯ ◯ Δ Δ X

Effects of the Invention

According to the present invention, there can be provided a filmcomprising polyethylene-2,6-naphthalenediacrboxylate as a principalcomponent, excellent in insulation characteristics without loweringprocessing characteristics, especially the characteristics duringhot-pressing and suitable as a dielectric for capacitors.

1. A polyester film for capacitors which is a biaxially oriented filmcomprising polyethylene-2,6-naphthalenedicarboxylate as a principalcomponent and having a thermal strain ratio RMD (150) of the film in themachine direction at a temperature of 150° C. of−1.5%≦RMD (150)≦0.0% when the thermal strain ratio RMD (T) in thetransverse machine direction at a temperature T° C. based on the lengthLMD (t) in the machine direction (MD) of the film at a temperature t° C.is defined byRMD (T)={[LMD (T)−LMD (35)]/LMD (35)}×100(%), further wherein thethermal strain ratio RTD (150) in the transverse direction of the filmat a temperature of 150° C. is−1.0%≦RTD (150)≦0.0% when the thermal strain ratio RTD (T) in thetransverse direction of the film at a temperature T° C. based on thelength LTD (t) in the transverse direction (TD) of the film at atemperature t° C. is defined byRTD (T)={[LTD (T)−LTD (35)]/LTD (35)}×100(%); and wherein the thermalstrain ratios RMD (210) and RTD (210) at a temperature of 210° C. are−3.5%≦RMD (210)≦0.0%−3.5%≦RTD (210)≦0.0%
 2. The polyester film for capacitors according toclaim 1, characterized in that the ratio of 5% straub strength in themachine direction of the film to the 5% strain strength in thetransverse direction of the film is 0.90 or more and 1.40 or less. 3.The polyester film for capacitors according to any one of claim 1 or 2,characterized in that the number of fly specks having an averagediameter exceeding 60 μm is 20 fly specks/m² or less.
 4. The polyesterfilm for capacitors according to claim 3, characterized in that 0.03 to2% by weight of calcium carbonate particles having an average particlediameter of 0.2 to 5 μm is contained and 0.03 to 1% by weight of platyaluminum silicate particles having an average particle diameter of 0.1to 2 μm is contained.
 5. The polyester film for capacitors according toclaim 4, characterized in that the number of coarse particles having amaximum diameter exceeding 35 μm is 10 coarse particles/m² or less. 6.The polyester film for capacitors according to claim 5, characterized inthat the film is produced according to a simultaneous biaxial stretchingmethod.
 7. The polyester film for capacitors according to any one ofclaim 1 or 2, characterized in that the number of fly specks having anaverage diameter exceeding 30 μm is 10 fly specks/m² or less.
 8. Thepolyester film for capacitors according to claim 7, characterized inthat two kinds of spherical silica particles different in averagediameter are contained.
 9. The polyester film for capacitors accordingto claim 8, characterized in that the two kinds of spherical silicadifferent in average particle diameter are spherical silica particles(A) having an average particle diameter of 0.5 to 3.0 μm and sphericalsilica particles (B) having an average particle diameter of 0.01 to 1.5μm, the spherical silica particles (A) and the spherical silicaparticles (B) each have a particle diameter ratio, major axis/minoraxis, of 1.0 to 1.2, the content of the spherical silica particles (A)is 0.03 to 1.5% by weight and the content of the spherical silicaparticles (B) is 0.05 to 2% by weight.
 10. The polyester film forcapacitors according to claim 9, characterized in that the film isproduced according to a simultaneous biaxial stretching method.
 11. Thepolyester film for capacitors according to any one of claim 1 or 2,characterized in that the number of coarse particles present in the filmand having a maximum diameter exceeding 40 μm is 10 coarse particles/m²or less.
 12. The polyester film for capacitors according to claim 11,characterized in that a porous silica particles and a spherical silicaparticles are contained, the porous silica particles have an averageparticle diameter of 0.5 to 5 μm, the spherical silica particles have anaverage particle diameter of 0.05 to 1.5 μm and less than the filmthickness, the spherical silica particles further have a particlediameter ratio, major axis/minor axis, of 1.0 to 1.2, the content of theporous silica particles is 0.05 to 2% by weight and the content of thespherical silica particles is 0.01 to 1% by weight.
 13. The polyesterfilm for capacitors according to claim 12, characterized in that anumber of fly specks having an average diameter exceeding 55 μm is 15fly specks/m² or less.
 14. The polyester film for capacitors accordingto claim 13, characterized in that the film is produced according to asimultaneous biaxial stretching method.